Design Variations-Glazing Area
Glazing Area in the Northern Zone (mostly heating)
As windows have improved considerably in the last
twenty years, very-high-performance windows can now equal or exceed the
performance of even an insulated wall over a complete winter heating season.
Consequently, the strategy of reducing window area to reduce energy use is no
longer as significant if highly efficient windows are used.
Total glazing area has a significant impact on heating energy use when poorly
insulating, single-glazed windows are used (Window A). This difference is
diminished with double-glazing and more so with low-E windows. With triple-glazed
low-E windows (Window F), the glazing area is not a factor in energy use at all.
Depending on the exact U-factor, SHGC, and climate, energy gains in the heating
season may be offset by losses in the cooling season. However cooling season
energy use can be further reduced by shifting the window area to preferred orientations and employing other cooling load
reduction strategies such as shading.
Window A |
Window B |
Window C |
Window D |
Window E |
Window F |
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Glazing |
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Frame |
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U-Factor |
1.16 |
.63 |
.49 |
.37 |
.34 |
.18 |
SHGC |
.76 |
.62 |
.56 |
.53 |
.30 |
.40 |
VT |
.74 |
.62 |
.58 |
.53 |
.50 |
.49 |
Glazing Area in the Central/North and Central/South Zones (heating and cooling)
In climates where there is both a significant
heating and cooling load, certain high-performance windows can effectively
reduce winter heat loss and summer heat gain. Total glazing area has a
significant impact on energy use when poorly insulating, single-glazed windows
are used (Window A). This difference is diminished with double-glazing which has
an improved U-factor but provides little help with the solar control (Windows B and C).
Low-E windows represent an even greater improvement but there is a notable
difference between high-solar-gain low-E (Window D) and low-solar-gain low-E
(Window E). In a climate with both heating and cooling loads, cooling season
savings with low-solar gain low-E are likely to outweigh heating season benefits
from high-solar-gain low-E (although this depends on the exact U-factor, SHGC,
and climate). Triple-glazed low-solar-gain low-E provides the best performance by
reducing winter losses even further (Window F).
This analysis indicates that increasing glazing area does increase energy use in this climate but it will not have
nearly as profound an impact when high-performance windows are used. In all
cases, cooling season energy use can be further reduced by shifting the window
area to preferred orientations and employing other cooling load reduction
strategies such as shading.
Window A |
Window B |
Window C |
Window D |
Window E |
Window F |
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Glazing |
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Frame |
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U-Factor |
1.16 |
.76 |
.47 |
.49 |
.37 |
.34 |
SHGC |
.76 |
.68 |
.33 |
.56 |
.53 |
.30 |
VT |
.74 |
.67 |
.53 |
.58 |
.53 |
.50 |
Glazing Area in the Southern Zone (mostly cooling)
The traditional approach to reduce heat gain is
to reduce the total glazing area. This strategy should only be used when less
efficient windows are used since new low-solar-gain low-E windows minimize
cooling load impacts. The figure illustrates the impact of two different amounts
of glazing on annual energy costs for a house in Phoenix, Arizona. In all cases,
the windows are equally distributed on the four orientations. With clear and
tinted glazings (Windows A, B, and C), doubling the glazing area from 300 to 600
square feet has a very significant impact on the cooling load. The annual energy
use for a house with low-solar-gain low-E glazing (Windows D and F) still
exhibits the same pattern, but the differences are not nearly as great in
relative or absolute terms. A high-solar-gain low-E glazing (Window E) in Phoenix
performs worse than the low-solar-gain low-E options and is not optimal in such a
hot climate.
This analysis indicates that increasing glazing area does increase energy use in
this climate, but it will not have nearly as profound an impact when
high-performance windows are used. Cooling season energy use can be further
reduced by shifting the window area to preferred orientations and employing other cooling load
reduction strategies such as shading.
Window A |
Window B |
Window C |
Window D |
Window E |
Window F |
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Glazing |
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Frame |
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U-Factor |
1.16 |
1.16 |
.76 |
.59 |
.37 |
.34 |
SHGC |
.76 |
.65 |
.68 |
.37 |
.53 |
.30 |
VT |
.74 |
.56 |
.67 |
.57 |
.53 |
.50 |
Note: The thermal performance properties of specific glazings and frames can vary depending on product design and materials. The results presented here are averages. Consult specific manufacturers for NFRC rated U-factors and SHGCs for products of interest. The annual energy performance figures shown here were generated using RESFEN for a typical new 2000 sq. ft. house with 300 sq ft of window area (15% of floor area) and 600 sq ft of window area (30% of floor area). The windows are equally distributed on all four sides of the house and include typical shading (interior shades, overhangs, trees and neighboring buildings). U-factor, SHGC, and VT are for the total window including frame. Energy use and savings between different window options will typically be higher for homes which are not as well insulated as typical new homes. The costs shown here are annual costs for space heating and space cooling only and thus will not correlate to utility bills. Costs for lights, appliances, hot water, cooking, and other uses are not included in these figures. The mechanical system uses a gas furnace for heating and air conditioning for cooling. These figures are based on typical energy costs for this region. Natural gas prices and electric prices are provided by the Energy Information Administration (www.eia.doe.gov).